second of arc: meaning, definition, pronunciation and examples
Very LowHighly Technical / Specialized Scientific
Quick answer
What does “second of arc” mean?
An extremely small unit of angular measurement, equal to 1/3600th of a degree or 1/60th of an arcminute.
Audio
Pronunciation
Definition
Meaning and Definition
An extremely small unit of angular measurement, equal to 1/3600th of a degree or 1/60th of an arcminute.
In practical terms, it is used to measure very precise angles or apparent sizes, such as the resolution of telescopes, the accuracy of mechanical systems, or the parallax of stars. Colloquially, it can describe anything extremely minute.
Dialectal Variation
British vs American Usage
Differences
No significant differences in meaning or usage. Both use 'arcsecond' interchangeably. Spelling of related terms (e.g., metre/meter) may differ.
Connotations
None beyond its technical precision.
Frequency
Equally rare and specialized in both dialects.
Grammar
How to Use “second of arc” in a Sentence
[Number] second(s) of arcaccurate to within [Number] seconds of arca resolution of [Number] seconds of arcVocabulary
Collocations
Examples
Examples of “second of arc” in a Sentence
verb
British English
- The instrument can second the angle with remarkable precision. (Note: This is a highly contrived, non-standard use for illustration only.)
American English
- The software seconds the star's position. (Note: This is a highly contrived, non-standard use for illustration only.)
adverb
British English
- The mount tracks arc-second precisely. (Note: This is a highly contrived, non-standard use for illustration only.)
American English
- It is aligned arc-second perfectly. (Note: This is a highly contrived, non-standard use for illustration only.)
adjective
British English
- The arc-second measurement was critical for the survey. (Note: Using the hyphenated form adjectivally.)
American English
- We need arc-second accuracy for this alignment. (Note: Using the hyphenated form adjectivally.)
Usage
Meaning in Context
Business
Virtually never used.
Academic
Used in specialized fields like astronomy, astrophysics, geodesy, optics, and precision engineering.
Everyday
Extremely rare. Might be encountered in advanced hobbyist contexts (e.g., amateur astronomy, high-end photography).
Technical
The primary context. Used to specify tolerances, resolutions, and measurements.
Vocabulary
Synonyms of “second of arc”
Neutral
Weak
Vocabulary
Antonyms of “second of arc”
Watch out
Common Mistakes When Using “second of arc”
- Saying 'second' alone in a technical context where ambiguity with time is possible.
- Omitting 'of arc' when first introducing the term.
- Confusing it with arcminutes (60 times larger).
- Incorrect pluralization: 'second of arcs' (correct: 'seconds of arc').
FAQ
Frequently Asked Questions
Yes, they are completely synonymous terms. 'Arcsecond' is more common in technical writing for brevity.
Yes, it is commonly abbreviated as "arcsec" or represented by the symbol (″), which is a double prime mark.
It comes from the system of sexagesimal (base-60) division. A degree is divided into 60 arcminutes, and each arcminute is further divided into 60 arcseconds. It is the 'second' division of the degree.
Primarily in astronomy (star positions, planetary sizes, telescope resolution), optics (lens quality), geodesy (Earth measurements), and high-end engineering/mechanics (precision alignment of parts).
An extremely small unit of angular measurement, equal to 1/3600th of a degree or 1/60th of an arcminute.
Second of arc is usually highly technical / specialized scientific in register.
Second of arc: in British English it is pronounced /ˌsɛkənd əv ˈɑːk/, and in American English it is pronounced /ˌsɛkənd əv ˈɑrk/. Tap the audio buttons above to hear it.
Learning
Memory Aids
Mnemonic
Think of a clock: a degree is like an hour, an arcminute is like a minute, and a second of arc is like a second—but for measuring angles in the sky or in machinery.
Conceptual Metaphor
ANGULAR MEASUREMENT IS TIME MEASUREMENT (degree/hour, arcminute/minute, arcsecond/second). PRECISION IS SMALLNESS.
Practice
Quiz
How many seconds of arc are in one degree?